Retaining Wall Calculator

Blocks, caps, base gravel, leveling sand, drainage stone, perforated pipe, filter fabric and geogrid for a segmental block retaining wall. It also works out the buried course, the setback, the dig and the labour, flags when the wall needs an engineer, and draws the wall section to scale.

Building a segmental block retaining wallA garden retaining wall under construction against a cut bank, with a finished upper terrace wall and planting behind. The trench holds a compacted crushed-stone base, shown cut away at the front with a thin layer of leveling sand under the first course. On the right the first course is being leveled: a rubber mallet taps the last block down until the bubble in the level centres. Each course above is set back from the one below, a new block is lowered onto the third course and a shovel tips clean drainage stone into the gap behind it. A black perforated drain pipe lies on the base behind the first course, filter fabric lines the cut bank, cap blocks are glued on at the finished end, and a plate compactor works the base further along the trench. Cap blocks, a pallet of wall blocks and a wheelbarrow of stone wait on the bank.

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Retaining wall reference

H1 COURSE6″SETBACK≈ 2× BLOCK DEPTHFINISHED GRADE, RETAINED SOILCAP GLUED WITH BLOCK ADHESIVE6–8″ COMPACTED SOIL OVER STONENON-WOVEN FILTER FABRICGEOGRID EVERY 2 COURSESCOMPACTED BACKFILL, 8″ LIFTS12″ CLEAN ¾″ DRAINAGE STONE4″ PERFORATED PIPE TO OUTLETSBURIED COURSE6″ COMPACTED ¾″ MINUS BASEUNDISTURBED SUBGRADE
Typical segmental retaining wall section, not to scale. Values are common trade practice for walls up to about 4 ft; the block maker's installation guide and an engineer's design take priority, and taller walls or walls with a slope, driveway or building above need an engineered design.

Base and first course

Buried depth
1 course or 10% of H
Base, compacted
6″ of ¾″ minus
Base width
≈ 2× block depth
Base past the face
6″ in front
Leveling sand
½″ max, optional
First course
level both ways
Setback
¾–1″ per course

Drainage

Drainage zone
12″ behind the block
Drainage stone
clean ¾″ crushed (#57)
Hollow cores
fill with the same stone
Drain pipe
4″ perforated, at the heel
Pipe outlets
every 30–50 ft
Filter fabric
non-woven, soil side
Soil over the stone
6–8″, compacted

Geogrid

Needed
over ≈ 3–4 ft or surcharge
Vertical spacing
every 2 courses
First layer
on top of course 1
Length
≥ 0.6 H, often 4 ft min
Strong direction
into the slope
At the face
to within 1″ of the front
Backfill
8″ lifts, compacted

Limits and checks

Garden block, unreinforced
≈ 2 ft
Standard units
≈ 3–3½ ft
Large units
≈ 3½–4 ft
Permit, engineer
over 4 ft or surcharge
Slope above
flatter than 3:1 for DIY
Terraced walls
gap ≥ 2× lower wall
Before digging
call 811

Safety Call 811 before you dig. Walls over about 4 ft, measured from the bottom of the base, or with a slope, driveway or building above usually need a permit and an engineer's design. A cut bank can collapse: do not work in a trench or under a vertical cut deeper than about 4 ft, lay it back instead. Blocks weigh 25–80 lb each; cutting them makes silica dust, so cut wet and wear a respirator, eye and hearing protection.

Full notes
  • The first course decides the whole wall. Set it on a compacted, level base, check every block front to back and side to side, and tap it down with a rubber mallet rather than packing material under it.
  • Bury at least one full course, or about 10% of the wall height if that is more. The buried course keeps the toe from kicking out and hides the base.
  • Drainage matters more than block weight: fill 12 in behind the wall and the hollow cores with clean ¾ in crushed stone, run a 4 in perforated pipe at the bottom and daylight it every 30–50 ft. Most failed walls failed because water pressure built up behind them.
  • Put non-woven filter fabric between the drainage stone and the soil, not against the back of the blocks, and finish the top 6–8 in with compacted soil so surface water runs over the wall instead of into it.
  • Backfill and compact in lifts of about 8 in, one or two courses at a time, using a hand tamper or a light plate within 3 ft of the wall. Never let a heavy machine near the back of a fresh wall.
  • Check the maker's height chart: garden blocks are for walls up to about 2 ft, standard units about 3 ft and large units about 3½–4 ft without geogrid. Taller walls, terraced walls closer than twice the lower wall's height, or anything with a slope, driveway or building above need an engineered design and usually a permit.

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